In recent years, there have been strong demands for automobiles to have a wider inner space for a high roof, wider sheets, and the like. Thus, ducts for automobiles are required to be compact and to smoothly flow air at a desired flow rate. Moreover, various ducts having different shapes are required for every type of automobiles.
In order to address these demands, related art ducts for automobiles have been made of a hollow molded product composed of high-density polyethylene, polypropylene and the like. The hollow molded product made from these materials can be easily formed into various shapes. In addition, the hollow molded product makes it possible to easily maintain a desired amount of air flow. Therefore, the hollow molded product is desirably used as ducts for automobiles.
However, noise generated by a compressor of an air conditioner and noise generated by an air flow passing through ducts are leaked out of a duct blowing outlet. In recent years, there have been demands for reducing these noises. However, the material for a related art molded product is a non-foaming resin such as a high-density polyethylene, and polypropylene. For this reason, the hollow molded product itself doesn't have a function for absorbing noise.
JP-A No. 2004-116959 has proposed a duct that can reduce noise generated by a compressor of an air conditioner and noise generated by an air flow passing through ducts. It is described that this duct has superior properties, such as a dew condensation preventive property, a heat-insulating property, a heat resistant property and a light weight property, as well as strength that is sufficient in practical use, and can be produced by using simple processes.
More specifically, the duct disclosed in that application is formed by using an intermediate blow-molded foam that has an apparent density in a range from 0.05 to 0.5 g/cm3, and a closed cell rate of 50% or more. Moreover, the inner surface of the hollow blow-molded foam has a surface hardness of 45 to 80 degrees.
As described in the '959 reference, the hollow blow-molded foam has an apparent density in a specific range and a closed cell rate in a specific range. Moreover, the hollow blow-molded foam has a surface hardness on its inner surface within a specific range. As a result, the duct made of such a hollow blow-mold foam has a superior sound absorbing property. Moreover, the duct makes it possible to reduce noise generated by a compressor of an air conditioner and noise generated by an air flow passing through ducts.
For example, a hollow blow-molded foam disclosed in the '959 reference is manufactured in the following method. As illustrated in FIG. 13, a foaming parison composed of a thermoplastic resin and a foaming agent is extruded from a die. The foaming parison is disposed in split mold blocks provided with a pressure-reducing pipe. Then, the foaming parison is sandwiched by the mold. While a gas such as air is being blown into the foaming parison, the split mold blocks are closed. As the split mold blocks are closed, the foaming parison is deformed along the shape of a cavity. Simultaneously, the foaming parison tightly adheres to the inner surfaces of the split mold blocks. When the split mold blocks are completely closed, a space (air passage) defined by walls of a hollow blow-molded foam is formed as illustrated in FIG. 14. In the closed state of the split mold blocks, the hollow blow-molded foam is cooled by using a cooling device not shown. The hollow blow-molded foam thus formed has an apparent density of 0.05 to 0.5 g/cm3 and a closed cell rate of 50% or more. The inner surface of the hollow blow-molded foam has a surface hardness of 45 to 80 degrees.
However, in some cases, a hollow blow-molded foam having a high expansion ratio (for example, an expansion ratio of 2.5 folds or more) and a high thickness (for example, thickness of 2.0 mm or more) is produced by using the above forming method. In this case, the thickness of the hollow blow-molded foam becomes thicker during the forming process.
However, in the case when the thickness of the hollow blow-molded foam becomes thicker during the forming process, the heat insulating property of the hollow blow-molded foam itself becomes higher. For this reason, upon cooling the hollow blow-molded foam, the cooling process sometimes fails to sufficiently cool the inside of each wall of the hollow blow-molded foam. In the case when the inside of each wall is not cooled sufficiently, withdrawing and warping tend to occur in the hollow blow-molded foam. As a result, the time required for cooling the hollow blow-molded foam needs to be prolonged. Moreover, after taking out the hollow blow-molded foam from the split mold blocks, the shape of the hollow blow-molded foam also needs to be corrected.
When the cooling process for the wall inside the hollow blow-molded foam is delayed, the inner cells of the wall are expanded to become bulky. As a result, cells are ruptured to sometimes cause a roughened inner surface of the wall. When the inner surface of the wall is roughened, a resistance to air (fluid) flowing through a space (air flow passage) defined by the walls increases. This causes a reduction in the flow-rate efficiency of a fluid that flows through the inside of the hollow blow-molded foam.